Flip chips, ball grid arrays (BGAs), wire bond pads, chip resistors and chip capacitors are examples of surface-mount devices, i.e., discrete circuit devices mounted to the surface of a circuit board, such as a printed circuit board (PCB), ceramic substrate, printed wiring board (PWB), flexible circuit, or a silicon substrate. These devices rely on solder connections to both secure the chip to a circuit board and electrically interconnect the device to conductors formed on the circuit board. The size of a flip chip is generally on the order of a few millimeters per side, while bond pads, chip capacitors and resistors are typically smaller. As a result, the conductors required for surface-mount devices are narrow, e.g., line widths of about 0.5 millimeter or less, and typically spaced apart about 0.5 millimeter or less.
Because of the small size of the solder connections, soldering a surface-mount device to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder using methods such as printing and electrodeposition. For smaller surface-mount devices, such as chip resistors and capacitors, the chip is soldered to its conductors by registering terminals formed on the chip with solder deposited on the conductors, and then reheating, or reflowing, the solder so as to form a "solder column" that metallurgically adheres and electrically interconnects the chip to the conductors, yielding what will be referred to herein as a solder connection. Mounting of flip chips and BGAs differ in that the solder is typically deposited on bond pads on the chip. Thereafter, the chip is heated above the liquidus temperature of the solder to yield "solder bumps." After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reflowing the solder, again forming solder connections.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the orientation of smaller surface-mount devices and the height of flip chip solder connections after soldering. As is well known in the art, smaller chips are prone to twisting and tilting during reflow as a result of the device floating on the surface of the molten solder, while controlling the height of flip chip solder connections after reflow is often necessary to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between a flip chip and its substrate, which may be termed the "stand-off height," is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and encapsulation materials between the chip and its substrate.
The position and height of a solder column of a discrete component are generally controlled by limiting the surface area over which the printed solder is allowed to reflow. As illustrated in FIG. 1, which shows a conductor 12 in longitudinal cross-section, the latter approach typically involves the use of solder stops 14, which can be formed by a solder mask or printed dielectric. The solder stops 14 are shown as extending widthwise across the surface 18 of the conductor 12, which has been printed or otherwise formed on a circuit substrate 10. A printed pad of solder 16 is shown as being adhered to the surface 18 of the conductor 12, as would be the case after solder has been printed and reflow soldered to the conductor 12. As is apparent from FIG. 1, the solder stops 14 delineate a rectangular-shaped area on the surface 18 of the conductor 12 over which the solder is able to flow during reflow. By properly locating the solder stops 14 on the conductor 12, the degree to which the molten solder can spread during reflow is controlled, which in turn determines the height of the solder connection and therefore the stand-off height of the component relative to the substrate 10.
Because the solder 16 is registered and soldered directly to the conductor 12, the conductor 12 must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor 12 through the formation of a metallurgical bond. In contrast, the solder stops 14 are intentionally formed of a nonsolderable material, meaning that a tin, lead or indium-based solder will not adhere to the material for failure to form a metallurgical bond. Upon reflow, the rectangular-shaped reflow area formed by the solder stops 14 on the conductor 12 yields a solder connection having a columnar shape between the component (not shown) and the conductor 12.
While solder stops are widely used in the art, trends in the industry have complicated the ability for solder stops to yield solder connections that exhibit adequate reliability for small discrete components such as wire bond pads, chip capacitors and chip resistors. Particularly, the trend is toward the use of low-melting, high-tin (e.g., 60Sn-40Pb) solder that is relatively brittle. Thermal cycle reliability problems can occur when a brittle solder solidifies against a solder stop used to contain the solder during reflow. Fatigue fracturing during thermal cycling tends to occur at the junction between the conductor, solder and solder stop.
Accordingly, it would be desirable if an improved method were available for controlling the region of a conductor over which solder can flow during reflow soldering of a device to the conductor, so as to achieve proper placement of the device while promoting the reliability and durability of the solder connection that mechanically secures the device to the conductor.